The hydrogenation of aromatic compounds is well known and has been disclosed throughout the art. In earlier processes when, for example, benzene was contacted with hydrogen in the presence of a hydrogenation catalyst at elevated temperatures and pressures there was typically good conversion of benzene to cyclohexane, however side reactions took place, such as cracking with the production of normal hexane and isomerization with the production of methyl cyclopentane. And, in the case of excessively high temperatures, the formation of C.sub.5 and lighter hydrocarbons was observed.
Where cyclohexane is used as an intermediate for the production of other chemicals, it is desirable to obtain the cyclohexane in as high purity as possible. It has been found that in order to do this the reaction temperature must be kept below a threshold where isomerization and cracking occur to a significant extent. In addition, complete hydrogenation is favored thermodynamically by a lower reaction temperature. For example, if the concentration of benzene is to be less than 100 ppm, the reaction must be conducted at less than 235.degree. C. according to thermodynamic calculations.
Hydrogenation is inherently exothermic, so a number of devices have been used to maintain a lower temperature. These include multiple catalyst beds with interbed heat exchange and cooling of the reactant stream and tubular reactors. A preferred process involved introduction of a mixture of benzene feed and cyclohexane product into the multiple catalyst bed unit with the introduction of product cyclohexane between the beds for the purpose of cooling the reactant stream. This effectively resulted in a low space velocity since only a small volume of material being passed through the bed was actually being hydrogenated.
SRI Report No. 713 Supplement B, January 1976, provides a review of a number of processes for hydrogenating benzene, along with important considerations involved in designing systems to accomplish hydrogenation.
In U.S. Pat. No. 3,202,723 there is described a process in which liquid phase is employed in the first reactor and vapor phase in the finishing reactor, where the catalyst in the first reactor comprises suspended Raney nickel.
U.S. Pat. No. 3,070,640, to Kellog, discloses a system wherein the main reactor is a tubular reactor filled with catalysts of progressive activities along the feed passage. The catalysts include nickel, platinum and such on alumina, silica and similar elements. No gain or loss of heat is allowed to occur in the finishing reactor.
A shaft reactor which has catalyst layers of progressively rising activities in the main reactor section is disclosed in British Pat. 1,008,666. Heat is removed by circulating fluid outside the shafts.
A two catalyst scheme is described in British Patent 1,104,275 where the benzene is first contacted with a platinum catalyst followed by a nickel catalyst. The process appears to take place in vapor phase.
In U.S. Pat. No. 3,767,719, to Texaco, there is disclosed a vapor phase tubular reactor containing nickel, platinum and palladium catalysts. The reactor is cooled with a mixture of cyclohexane and feed. The heated medium is flashed and separated into vapor and liquid. The vapor is charged to the catalyst zone while the liquid recirculates to cool.
A two stage reactor is disclosed in U.S. Pat. No. 3,796,764, assigned to Texaco Inc. The first stage comprises nickel on alumina and the second stage comprises platinum on alumina. That process required a cyclohexane diluent for the benzene feed to control the exothermic reaction. The first catalyst is less active than the second which helps to control the heat generated. Moderation of the catalyst activity was accomplished by using a lower metal concentration on the catalyst. The benzene/hydrogen molar ratio was 4:1 to 15:1, the temperature range was 350.degree. F.-680.degree. F. and the pressure was about 300 psig to about 600 psig.
It would be a distinct advance in the art if a process were available which required no diluent circulating in order to control the exothermic reaction. Additional improvements would include improved selectivity for cyclohexane and reduced formation of the by-product methylcyclopentane. A process where the benzene feed remained in liquid phase, along with other hydrocarbons in the process, and where no separation is required would definitely have commercial advantages.